This invention relates generally to quartz parts used in thermal reactors as used in the semiconductor industry and, in particular, to chemical vapor deposition (CVD) reactors.
In chemical vapor deposition (CVD) reactors a film is deposited on semiconductor substrates through a chemical reaction of one or more reactants supplied in the gas phase and brought into contact with the substrates. However, a film is not only deposited on the substrates but also on reactor parts that are at high temperature. Whereas new wafers are processed in each subsequent run, such that each wafer receives the deposited film only once, the reactor parts stay continuously in place and they receive multiple depositions. After a number of accumulated depositions, the films on these reactor parts start to crack and to flake; as a consequence the wafers will be contaminated, which is undesirable.
Therefore these parts need to be cleaned before the flaking results in unacceptable particle formation. In particular, the susceptor in case of a single wafer reactor, or the wafer boat in case of a batch furnace, are critical because they are in close proximity of the wafers. When the reactor parts cycle in temperature, there is an extra risk because differences in thermal expansion coefficient between the reactor part and the deposited film result in thermal stresses. It will be clear that removing the cumulative deposited film from the reactor parts is a cumbersome job and results in down time of the reactor during which it cannot be used for production. Typically, these reactor parts are made of quartz but also silicon carbide is regularly used. The silicon carbide is often impregnated with free silicon.
Providing a coating on the reactor part prior to exposing the reactor part to the chemical vapor deposition process in the reactor to which it belongs could help to solve these problems. In particular, improving the adherence between the reactor part and the CVD coating produced in the reactor by an intermediate coating would be beneficial.
In the prior art it has been proposed to coat quartz reactor parts by chemical vapor deposition for different purposes. U.S. Pat. No. 4,389,967 suggests coating the friction-prone surfaces of a wheeled quartz boat with a CVD nitride film to prevent seizure from occurring during diffusion processes. U.S. Pat. No. 4,428,975 suggests providing the quartz tube with a CVD polysilicon coating to prevent streaks from developing on the wafers in a CVD nitride process. U.S. Pat. No. 4,522,849, suggests coating a quartz tube with CVD boron nitride to prevent contaminants from diffusion through the quartz. However, the problems solved in these prior art documents are different from the problem described here. Furthermore, it can be expected that in all cases where a film is deposited on a reactor part of completely different properties, the adherence between the film and the reactor part will not be optimal. Because quartz is a relatively soft material, particularly at the high temperatures used in the reactor, the surface of the quartz can easily be damaged by a film of different properties deposited on the quartz surface and cracks in the quartz can easily propagate.
It is an object of this invention to provide a method of pre-treatment of reactor parts used in chemical vapor deposition reactors in the semiconductor industry. The treatment should increase the cumulative deposited film thickness that can be received by the reactor part before contamination of wafers processed in said reactors exceeds acceptable limits. In this way the number of process runs that can be performed between cleaning operation can be increased, resulting in a more economical operation.
It is another object of the invention to provide a method of coating a reactor part with a film that renders the surface of the reactor part less vulnerable to damage, thereby reducing damage to the reactor part during the coating process.
It is a further object of the invention to provide a method of pre-treatment of reactor parts used in Chemical Vapor Deposition reactors that can be performed in-situ, within such reactors, without the use of a separate pre-treatment facility.
In accordance with one aspect of the invention, prior to exposure of the reactor part to the chemical vapor deposition process, the reactor part is heated to a temperature of at least about 800xc2x0 C. and exposed to a nitrogen-comprising gas during some time. This process results in thermal nitridation of the surface of the quartz- or silicon-comprising reactor part. According to a further aspect of the invention, this pre-treatment is performed in-situ, within the reactor.
According to a preferred embodiment, the nitrogen-comprising gas comprises NH3. Because the surface material of the reactor part is converted into silicon nitride or into silicon oxynitride, the nitrogen-rich surface film is highly interwoven with the bulk material of the reactor part and a perfect adherence is obtained. Furthermore, a gradual transition occurs from the bulk composition to the surface film composition, avoiding unwanted damage to the bulk material of the reactor part.